OPTICAL PROPERTIES OF GRAPEVINE LEAVES: REFLECTANCE, TRANSMITTANCE, ABSORPTANCE AND CHLOROPHYLL CONCENTRATION

Leaf absorptance (A), transmittance (T) and reflectance (R) of visible solar radiation strongly correlate with chlorophyll concentration in a number of plant species, however, little is known about the optical properties of grapevine leaves. Consequently, the objective of this study was to evaluate the optical properties of intact leaves for estimating chlorophyll concentration in Cabernet Sauvignon, Merlot and Tempranillo vaieties. Leaves from Cabernet Sauvignon, Merlot and Tempranillo vines were collected at San Antonio de las Minas, Baja California, Mexico, in 2009, and A, T and R determined with a spectroradiometer. While leaf age played a significant role on the leaf chlorophyll concentration, the chlorophyll a+b concentration vs. R, T and A (evaluated from 400 to 700 nm) in the leaves of the three varieties studied followed an exponential relationship. There was a clear difference in the chlorophyll content, and the R, T, and A spectra among leaves of different ages in all three varieties studied. Reflectance values at wavelengths (lambda) >750 nm increased as the age of the leave increased while lambda values at approximately 550 nm remained relatively constant. The mean A values of the Merlot (0.80 +/- 0.07), Cabernet (0.82 +/- 0.05) and Tempranillo leaves (0.78 +/- 0.08) studied were below the assumed A value of 0.84 for vascular plants. Our observations suggest that while the 0.84 A value is acceptable for estimating absolute electron transport rates (ETR) in mature grapevine leaves, relative ETR values must be reported in young leaves. Band ratios were developed to optimize the evaluation of canopy area, canopy water status, canopy chlorophyll concentration and others using R measurements. Optimum band ratios were generated by dividing R at the best fit lambda by R at each lambda through the 400 to 850 nm range and regressing total chlorophyll concentration vs. ratio value.